The redtail shark isn't the compatibility issue,it's the puffer's.Besides the fact that Angels need completely different water parameters than puffer's,their temperament is a stark contrast as well.You will hear people say that they kept their puffers with every fish under the sun.However,read any worthy book on species compatibility and you will see that puffer's can be waspish,territorial,and just plain belligerent.There are species you can keep with them,just do some research.Also,though they will fair better in a brackish set-up,it is not totally necessary.Depending on the species,I have kept them alive and well using Seachem's Cichlid Lake Salts and buffers.Good luck...